Preventing and detecting pipeline corrosion is a major concern in the pipeline industry. Two approaches to mitigating corrosion are applying pipeline coatings and subjecting pipelines to cathodic protection.
With regard to pipeline coatings, various factory-applied and field-applied coatings are available to cover the outer surface of the metallic pipeline wall. Examples of factory-coating materials are fusion-bonded epoxies, and high-density polyethylene and urethanes. Examples of field-applied coatings are spray coatings such as epoxy, urethane and zinc, various waxes and petrolatum, bitumen-based coatings. Various tapes or other coverings of these or other materials may also be applied to the pipeline.
Cathodic protection may be passive or active. Passive cathodic protection is achieved by electrically connecting the pipeline to another more easily corroded metal to act as the anode of a simulated electrochemical cell. Active cathodic protection uses impressed currents; a current is applied to the conductive pipe wall to force it to act as a cathode.
However, even the best of today's corrosion avoidance techniques do not prevent corrosion at some point. Coatings can degrade, creating areas where the coating material is partially or wholly disbonded from the pipe, limiting protection from corrosion. Also, pipeline installation, especially installation with horizontal direction drilling, can result in extensive coating damage.
Buried pipelines are not easily accessible for corrosion detection. Thus, special techniques have been developed for detecting coating defects and other corrosion without need to expose the pipe surface.
For pipelines that are installed using conventional trench installation, various technologies have been developed for detecting corrosion from the surface above the pipeline. However, pipelines installed using horizontal directional drilling are often installed in this manner to run under features, such as rivers or structures, where trenching is not practical. Thus, there is limited access to the surface above the pipeline. Therefore, pipelines installed using horizontal drilling may require special methods and devices for defect detection.